The article presents a new scheme of an optical microscope. The proposed scheme contains two channels: one works with a wide field of view, the other forms a high-resolution image. The simultaneous fulfillment of these conditions significantly increases the information capacity of the system. The proposed scheme overcomes the limitations imposed by the Lagrange invariant. In addition, the presented work solves the relevant task of optical instrumentation: using one device,it becomes possible to simultaneously obtain an image of an object in two scales, one of which gives a detailed image of microstructures, and the other shows the state of the area around the part of the object being studied in detail. This time synchronization is especially important for studying biological objects. The design features of the optical scheme make it possible to abandon the refocusing of the objective, which is necessary in biological microscopes when the magnification is changed. Thus, the proposed system solves not only scientific and technical tasks, but also improves the ergonomics of work in microscopy.
Microscope is one of traditional types of optical devices used to research of
microstructures. Working with a microscope is closely related to the peculiarities of how
operator perceives the object under research. It is known that before the formation of
a sensory image in a person's perception, there is a detection, distinction and
identification of an object (in whole or in its details). It should be noted that systems of
machine vision try to implement a similar algorithm of working with objects [
The purpose of using optical devices is empowerment of human eye. Each of the mentioned stages of perception assume the discrimination of smaller elements; there1 Copyright © 2020 for this paper by its authors. Use permitted under Creative Commons License Attribution 4.0 International (CC BY 4.0). fore, the optical device should help to increase the resolving power of the operator's eye. In visual instruments, this is associated with an increase in magnification. The traditional solution of this purpose in microscopy is using of a revolver with several micro-objectives in the device.
Nowadays, high requirements on optical devices in terms of the quality of the
resulting image are imposed. Also, consumers often want to observe as many objects as
possible at the same time. However, the ability of an optical system to provide a
highresolution image in a wide field is impossible on the basis of the Lagrange invariant
[
Changing the magnifications in the microscope takes away a significant part of the information about the object: the operator is deprived of the opportunity to observe what is locating around the research field, which is always subject of great interest in the case of living biological samples.
In addition, the change in magnifications disrupts the continuity of the observation
process, therefore, does not allow use operator's psychophysical abilities effectively.
Also, this action takes time to refocus the system, which can be critical when studying
samples that change their appearance over time: they can be lost from the field of
view or, in the case of living cells that are sensitive to light, irradiate them in vain,
wasting their life resource only to configure the system [
As a result, there is a contradiction: the operator is either limited in resolution but retains the possibility of indirect observation of the rest of the field, or acquires the ability to see details with high resolution, but only in a very small area. Average magnification with an average field of view is a compromise solution that includes the disadvantages of both options rather than their advantages.
The purpose of this work is to design an optical scheme of a microscope that allows observing objects simultaneously in a wide field and with a high resolution. The use of such systems will make it possible to eliminate the need of lens refocusing arising from a sharp decrease in the depth of field and will also provide the possibility of detecting and identifying the structures under research without reconfiguring of visual instrument. 2
As stated in the Abbe diffraction theory, the resolution of a microscope is directly
related to its numerical aperture and wavelength [
= 2 (1) In the case of work in the visible range, the resolution of the microscope entirely depends on its numerical aperture. However, as mentioned above, observation of an image at once in a wide field and with a high aperture is limited according to the Lagrange invariant.
Optical Design of a Dual Channel Microscope 3
In order to overcome this restriction, it was decided to develop a dual-channel
optical system. Such systems are widely used in optical instrumentation and in
microscopy in particular. Examples of instruments with observation in two channels are
comparative microscopes and forensic microscopes. For instance, the LEICA FS CB [
An example of two-channel devices, both channels of which are directed to the
same object, are microscopes using interference methods. They have found wide
application in optical coherence tomography [
Wide-field high-aperture microscopes are also known [
The prototype of the development is basic optical design of the research
metallographic microscope MIM-7 [
The illumination channel includes a light source 1, the rays from which are directed by the collector 2 to the mirror 3. Reflecting from the mirror, the light beam sequentially encounters on its way a light filter 4, aperture diaphragm 5, lens 6 (and polarizer 21, if it is included into system), a photo shutter 7 and a field diaphragm 8. The pentaprism 9 re-reflects the beam, which is then refracted by the lens 10 and falls on the semitransparent reflective plate 11.
The scheme of the lighting channel is completed by microlens 12 and illuminated research subject 13. At the same time, at this place the observation channel begins: in a reverse beam path from sample through the microlens 12 and the beam-splitting semitransparent plate 11. After the beam-splitting plate 11 the light passes through the plug-in analyzer 20 and is refracted by achromatic lens 14. Depending on the selected method of observation (visually or with a photodetector) operator can change the position of the mirror 18. When the mirror 18 is included into the system, the beam of rays enters the eyepiece. To photograph the object, the mirror is removed from the system, and the beam of rays continues to move through the photo eyepiece 15, is reflected by the mirror 16 and falls on the image receiver 17. As mentioned above, the lens 12 provides two channels. It is designed as objective with infinity tube, which allows the division of light without adding aberrations. However, such a lens is not designed for use with a cover glass that is necessary for the research of biological objects, which, at high apertures, will add large spherical aberration. Furthermore, there is only one observation channel in the optical scheme of a metallographic microscope. The illumination in this system is in reflected light, while the research of biomaterial requires illumination in transmitted light. Based on the foregoing, although the optical scheme of metallographic microscope MIM-7 is the basis for the optical system under development, its basic optical design is far from being a complete analogue. 4
The proposed scheme of a dual-channel microscope is shown on Figure 3 and consists of a frontal, wide-field and high-aperture objectives, a beam splitter and optical image receivers.
The front lens is designed for simultaneous wide field and high aperture operation. Its presence removes the need for refocusing on the subject, which usually occurs with changing the magnification in laboratory microscopes.
A beam splitter separates the light into two channels. To minimize aberrations, it
should have an aperture diaphragm. Moreover, a parallel beam of rays should fall on
the beam splitter, since in case an oblique beam falls on the beam splitter, intractable
coma and astigmatism will appear in the system [
Figure 4 shows the layout of dual-channel optical microscope in thin components.
The wide-field channel magnification is 4x, which corresponds to a number of
standard microscope magnifications. The numerical aperture in this channel is 0.1. The
second channel is designed for 40x magnification with an aperture of 0.5. Working at
this aperture will allow viewing objects with a size of 0.55 μm, according to (1).
It is proposed to implement illumination in a dual-channel microscope according to
Koehler (see Fig. 5) [